Sperm tail differentiation in the nudibranch mollusc Hypselodoris tricolor (gastropoda,opisthobranchia)

The sperm axoneme of Hypselodoris tricolor forms from a single centriole that is located initially beneath the plasma membrane and then migrates to the nuclear surface. A conspicuous centriolar adjunct-like formation is present in the neck of midspermatids, but it becomes very reduced at the end of spermiogenesis. In spermatocyte and spermatid mitochondria, intracristal bodies originate from the accumulation of a dense material in some cristae. From our observations and foregoing reports, it may be concluded that the process of sperm tail differentiation in opisthobranchs resembles that in pulmonates, whereas it differs in many respects from that occurring in prosobranchs. The appearance of intracristal bodies in modified mitochondria seems to be a special feature of spermatogenesis in the opisthobranchs that does not occur in the two other groups of gastropod molluscs.     

Basal chromodorid sperm ultrastructure (Nudibranchia, Gastropoda, Mollusca)

The relationship between three genera considered basal in the Chromodorididae (Cadlina, Tyrinna, Cadlinella) has not yet been resolved by traditional morphological means. Here we examined the sperm ultrastructure of Tyrinna nobilis, Tyrinna evelinae, Cadlina flavomaculata and Cadlina cf. nigrobranchiata, with the expectation of finding phylogenetically informative characters. No Tyrinna or Cadlina species showed sperm similarities to Cadlinella. Both Cadlina species and Tyrinna nobilis (but not T. evelinae) exhibited coarse striations in the acrosomal pedestal. The putative fibers that occurred between the coarse striations of the pedestal are condensed into a layer in Cadlina and Tyrinna, but not in other species that also have coarse striations (Gymnodoris), and may constitute evidence for a close relationship. Tyrinna evelinae possessed fine acrosomal striations, which was shared with other Chromodorididae, Actinocyclidae and the cryptobranchs Rostanga and Aphelodoris. We also examined the sperm ultrastructure of ‘Chromodoris’ ambiguus, an animal which has shown molecular affinities to species of Cadlina, and not Chromodoris. The sperm of ‘C.’ ambiguus did not exhibit the typical Cadlina characteristics, but also showed important differences to other investigated Chromodoris species.     

Zur Kenntnis der mesopsammalen GattungPseudovermis (Gastropoda,Nudibranchia)

The geographic distribution of the mesopsammobiont gastropod genusPseudovermis is reconsidered.P. papillifer andP. schulzi were found for the first time in the Northern Adriatic near Rovigno.P. kowalevskyi andP. boadeni are described. A taxonomic survey and a determination key are presented for allPseudovermis species known thus far.     

Comparative studies on the histology of the ovotestis in Hypselodoris tricolor and Godiva banyulensis (Gastropoda,Opisthobranchia), with special reference to yolk formation

The histology of the ovotestis was studied by light and electron microscopy in two nudibranch gastropod species. While in Hypselodoris tricolor the ovotestis is intimately associated with the digestive gland tissue, the large gonadal mass of Godiva banyulensis is placed freely in the haemocoele. This fact results in great histological differences between both species. As is common among Mollusca, the immature yolk granule in Hypselodoris and Godiva presumably originates from membrane-rich cytoplasmic inclusions, which we have termed dense multivesicular bodies. Such inclusions consist of an outer membrane enclosing membrane remnants and a granular, electron-dense material. These elements are accumulated and mixed in the center of the dense multivesicular body and could be actually transformed into the paracrystalline core of the immature yolk granule, the cortex of which is made up of part of the central accumulation materials that have not spread into the crystal. During vitellogenesis, some mitochondria are subjected to a process of transformation affecting mainly their inner membrane (including mitochondrial cristae) and matrix. However, the conversion of modified mitochondria into yolk precursors, as reported for other gastropod species, could not be determined with absolute certainty on the basis of our observations on static material. The mature yolk granule consists of a central paracrystalline core, similar in structure to that of the immature yolk granule, and a peripheral membranous cortex, which seems to spread centripetally, thus permitting the crystal to grow. The cortical material consumed in synthesizing the central core appears to be restored by addition of degenerative mitochondria to the yolk granule surface.     

Nuclear morphogenesis during spermiogenesis in the nudibranch molluscHypselodoris tricolor (Gastropoda,opisthobranchia)

An electron microscope study was carried out on Hypselodoris tricolor spermatids to describe the development of the nuclear morphogenesis and investigate the possible cause(s) of the change in the shape of the spermatid nucleus during spermiogenesis. Three different stages may be distinguished in the course of the nuclear morphogenesis on the basis of the morphology and inner organization of the nucleus. Stage 1 spermatid nuclei are spherical or ovoid in shape and the nucleoplasm finely granular in appearance. Stage 2 nuclei exhibit a disc- or cup-shaped morphology, and the chromatin forms short, thin filaments. During stage 3, a progressive nuclear elongation takes place, accompanied by chromatin rearrangement, first into fibers and then into lamellae, both formations helically oriented. A row of microtubules attached to the nuclear envelope completely surrounds the nucleus. Interestingly, the microtubules always lie parallel to the chromatin fibers adjacent to them. Late stage 3 spermatids show the highest degree of chromatin condensation and lack the manchette at the end of spermiogenesis. Our findings indicate the existence of a clear influence exerted on the chromatin by the manchette microtubules, which appear to be involved in determining the specific pattern of chromatin condensation in Hypselodoris tricolor.     

On the phylogeny of the Arminidae (Gastropoda, Opisthobranchia, Nudibranchia) with considerations of biogeography

In the past, the different genera of the family Arminidae have been diagnosed (mostly) on the basis of plesiomorphic characters and therefore their monophyly is questionable. The Arminidae are characterized by the autapomorphies 'possession of marginal sacs' and 'rhinophores with longitudinal lamellae'. The genus Heterodoris is excluded from the family. Two possible hypotheses about the phylogenetic relationships within the Arminidae are presented.
The Arminidae probably originated in the western Pacific, near Japan and have distributed in a westerly direction. The most primitive species ( Dermatobranchus ) are restricted mainly to the western Pacific region while the more derived species ( Armina ) have a world-wide distribution.     

Bau und Funktion des Cnidosacks der Aeolidoidea (Gastropoda Nudibranchia)

Zusammenfassung Die Histologie des Cnidosackes wird an 14 adulten Aeolidoidea-Arten aus dem Golf von Neapel (Mittelmeer) und aus Roscoff (Ost-Atlantik) in der Ruhe und während des Cnidenausstoßes lichtmikroskopisch geprüft und an einer Spezies,Cratena peregrina, elektronenmikroskopisch untersucht.Die Morphologie des Nesselsackes stimmt bei allen 14 Arten ungefähr überein. Stets handelt es sich um ein distal blind geschlossenes, meist ellipsoides, von einschichtigem entodermalen Epithel ausgekleidetes Organ, das durch einen Porus oder engen Gang mit dem Hepatopankreas in offener Verbindung steht. Eine permanente distale Nesselsacköffnung in Form eines von Epithel ausgekleideten Ausführganges fehlt stets.Das Epithel des Nesselsackes besteht aus Cnidophagen und sog. interstitiellen Zellen. Oft läßt sich eine proximale Zone embryonaler Zellen von einer distalen Differenzierungszone unterscheiden, an die sich am Kolbenende wiederum eine Region embryonaler Zellen anschließt. Je kleiner der Nesselsack ist, destoweniger Zellen bilden die einzelnen Zonen. Die Cnidophagen der Differenzierungszone phagocytieren die Nesselkapseln und speichern sie langfristig, wobei Kern und Cytoplasma der Cnidophagen keinerlei Degenerationserscheinungen zeigen. Die Cnidocyte wird immer verdaut.Feinstrukturell zeigen die Cnidophagen einen voll aktiven Kern mit großem Nukleolus, ein starkes basales Labyrinth, große Mitochondrien, einen großen Golgiapparat und Vakuolen mit charakteristischen Inhaltskörpern. Perikaryell findet sich ein geringes rauhes ER, apikal glattes endoplasmatisches Reticulum. Die morphologisch voll intakten Nesselkapseln (Cleptocniden) liegen in einer von einer Einheitsmembran umschlossenen Vakuole.Die Anordnung der Cniden in den Cnidophagen ist für die einzelnen Schnekkenarten charakteristisch, z.B. parallel beiAeolidia papillosa, bouquetartig beiTrinchesia granosa. Die Entladungspole zeigen zum Nesselsacklumen. Alle Cniden werden in den Cnidophagen in unexplodiertem Zustand gespeichert und bleiben langfristig funktionsfähig. Bei vielen Arten werden nur bestimmte Cnidentypen des Futterhydroiden-Cnidoms gespeichert. Wie diese Selektion erfolgt, ist ungeklärt.Im Falle eines Cnidenausstoßes zeigen alle Arten an präformierte Stelle einen Riß durch die zuvor intakte Kolbenepidermis und durch das Cnidosackepithel. Diese präformierte Perforationszone wird charakterisiert durch Fehlen von Cnidosackmuskulatur und oft durch undifferenzierte Nematophagen. Die Cniden werden innerhalb der intakten Nematophagen ausgestoßen. Erst im Meereswasser platzen die Zellwände und die Cniden explodieren. Nach der Cnidenabgabe wird der Riß durch Epidermiszellen und die undifferenzierten Zellen der Mitteldarmdrüse wieder verschlossen. Der Cnidenausstoß erfolgt bei gesunden Schnecken nur auf einen kräftigen mechanischen Reiz hin. Eine langsame Cnidenabgabe über Tage hinaus oder eine spontane, sporadische exkretionsartige Ausschüttung wurden nicht beobachtet.

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Structure and function of the cnidosac of the aeolidoidea (gastropoda nudibranchia)

Summary The cnidosac has been investigated in 14 species of Aeolidoidea from the Gulf of Naples and from Roscoff with the light microscope and in one species,Cratena peregrina, with the electron microscope.The morphology of the entodermal cnidosac is similar in all species. The cnidosac is more or less an ellipsoid or spindle shaped organ, which is connected with an open pore or a narrow duct with the hepatopancreas — and which is always closed at its distal end. A permanent, epithelial cnidopore was not observed.The single-layered epithelium of the cnidosac is mainly composed of nematophages (= cnidophages). At the entrance of the cnidosac and at its distal end there often occurs a zone of undifferentiated cells, believed to serve for regeneration of lost cnidophages. The cnidophages phagocytize cnidocysts (nematocysts) and may store it for a long time. The nematocyst and the nematophag remain intact, whereas the nematocyt always becomes digested.The nematophages are characterized by large mitochondria, large golgi complexes, by a basal labyrinth, an active nucleus with a large nucleolus and by vacuoles containing tubular bodies. Pericaryell, a week rough endoplasmic reticulum, is found while in the apical cell region the endoplasmic reticulum is smooth and appears empty. The nematocysts possess an intact fine structure and lie within a vacuole. The unit membrane of the vacuole encloses the nematocyst very closely.The arrangement of the cnidocysts within the cnidophages is species-specific, p.e. the nematocysts lie parallel to one another inAeolodia papillosa, while they are arranged in a circle inTrinchesia ilonae or like a bouquet inTrinchesia granosa. Within the cnidophage the cnidocysts are stored unexploded and remain functional for a long time. Many species store only certain types of nematocysts of the cnidom of the hydroid. The mechanism of this selection is unknown.When the nematocysts are ejected the epidermis and the epithelium of the cnidosac burst at a preformed zone. This region is characterized by being very high and slender, but otherwise normal epidermal cells and often by undifferentiated nematophages. Muscles are absent in this region. The nematocysts are ejected within the nematophages. The cell membrane bursts only in the sea water and the cnidocysts explode. The cleft is closed by contraction, which bring the epidermal cells of the wound border and the undifferentiated cells of the cnidosac epithelium close together. The nematocysts are usually ejected only on appropriate violent mechanical stimulation. A slow, continuous ejection lasting for several days or a spontanuous, repeated ejection as a natural modus of excretion have never been observed.

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Mit dankenswerter Unterstützung durch die Deutsche Forschungsgemeinschaft und die Stiftung Volkswagenwerk. Wir danken ferner für die guten Arbeitsmöglichkeiten an der Stazione Zoologica di Napoli, am Institut Biologique de Roscoff und am Institut Arago in Banyuls     

A taxonomic revision of Paradoris sea slugs (Mollusca, Gastropoda, Nudibranchia, Doridina)

Paradoris , traditionally a generic taxon of discodorid sea slugs, is revised for the first time. One hundred and thirty specimens were examined, including all types and most of the nontype specimens available. New records for Paradoris are provided: South Africa, Tanzania, Seychelles, Western Australia, Thailand, Philippines, shallow waters of New Caledonia, southern Japan, and Hawaii. The individual variation of all taxonomic characters is thoroughly evaluated. Prior to the present study, Paradoris included 15 nominal species: 12 valid names and three synonyms. Three additional names, Discodoris erythraeensis , D. lora , and D. cavernae , are re-allocated to Paradoris , based on phylogenetic analysis. A phylogenetic diagnosis is provided for Paradoris : in particular, two new synapomorphies are described. Eight species names are regarded as valid: P. araneosa , P. dubia , P. erythraeensis , P. indecora , P. liturata , P. lopezi , P. mulciber , and P. tsurugensis . However, most of these species are poorly known, i.e. from very few specimens, and their taxonomic status might change when more individuals are available. P. lora is regarded as a nomen dubium . Six new synonymies are proposed, and explained by the fact that: (1) species names were created for one or a few specimens, without considering individual variation; (2) authors have not worked within a phylogenetic framework and have created new species names without considering all the existing species names already available within Paradoris . Three new morphospecies are described, but not formally named because their taxonomic status is still uncertain for several reasons (e.g. lack of knowledge of individual variation for some critical features).  © 2006 The Linnean Society of London, Zoological Journal of the Linnean Society , 2006, 147 , 125–238.     

Phylogenetic comparison of spicule networks in cryptobranchiate dorid nudibranchs (Gastropoda, Euthyneura, Nudibranchia, Doridina)

Many dorid nudibranchs possess large numbers of calcareous spicules in their mantle, gill, rhinophores and foot. However, the arrangements of these structures and their differences among taxa are poorly known. Spicule networks were stained with Alizarin red and compared among 12 species of cryptobranchiate dorid nudibranchs and four outgroups. Three general types of networks were found: a cobweb-like, unbraced framework of one or few spicules per side; a ramifying system of thick, spiculated tracts; and a lattice-like arrangement of distinct radial and circumferential tracts. The Discodorididae species investigated shared a cobweb-like network and papillae supported by a ring of spicules, while the Porostomata showed consistent characters leading to a lattice-like network with larger spicules in the central notum. The Dorididae studied were not cohesive, but each species shared characters with the aforementioned groups. Therefore, spicule network form may provide new characters to help resolve the phylogeny of Doridina.     

Phyllidiidae (Nudibranchia,Heterobranchia, Gastropoda): an integrative taxonomic approach including chemical analyses

Organisms Diversity & Evolution - Members of the widely distributed and common nudibranch family Phyllidiidae are often easily spotted in the marine environment because of their conspicuous...     

Die Gattung Bathydoris Bergh, 1884 (Gnathodoridacea) im phylogenetischen System der Nudibranchia (Opisthobranchia, Gastropoda)

The genus Bathydoris Bergh, 1884 (Gnathodoridacea) in the phylogenetic system of the Nudibranchia (Opisthobranchiu, Gastropoda) The position of the genus Bathydoris Bergh, 1884 in the phylogenetic system of the Nudibranchia is discussed. Doridoxa Bergh, 1900, a genus which is assigned to the taxon Gnathodoridacea together with the genus Bathydoris, is taken out from this taxon, lacking snapomorphies. Doridoxa is considered to be a sister grou of the Euctenidiacea, with the transrormation of the right digestive gland into a caecum as the only synaomorphy with the latter. The Gnathodoridacea with the one enus Bathydoris is removed from the taxon Doridacea and considered to be the sister group or the latter. Gnathodoridacea and Doridacea form the taxon Euctenidiacea with the synapomorphy “dorsomedial position of anus and nephroproct”.     

Ultrastructure of the larval eyes of Hermissenda crassicornis (Mollusca: Nudibranchia)

Hermissenda crassicornis, an aeolid nudibranch mollusk, possesses a simple eye composed of five photoreceptors, pigmented supporting cells, and a lens. Veliger larvae were studied using light and electron microscopy to examine the structure of the larval eye and compare it with that of the adult.Larvae 21 and 25 days old were found to possess smaller, less differentiated eyes than those of the adult. Ultrathin serial sections of 25-day-old animals showed three photoreceptor cells in the larval eye as compared with five in the adult eye. Each photoreceptor cell occupied a specific position within the eye. One of the three photoreceptor cells, cell l, contained small, clear vesicles, termed photic vesicles by R. M. Eakin and J. L. Brandenburger (1970, J. Ultrastruct. Res.30, 619–641), that are characteristic of gastropod photoreceptors. The possible significance of cell I is discussed in terms of developmental sequence. No optic nerve was found to be present at either of the stages examined.     

Comparative morphology and evolution of the cnidosac in Cladobranchia (Gastropoda: Heterobranchia: Nudibranchia)

Background

A number of shelled and shell-less gastropods are known to use multiple defensive mechanisms, including internally generated or externally obtained biochemically active compounds and structures. Within Nudipleura, nudibranchs within Cladobranchia possess such a special defense: the ability to sequester cnidarian nematocysts – small capsules that can inject venom into the tissues of other organisms. This ability is distributed across roughly 600 species within Cladobranchia, and many questions still remain in regard to the comparative morphology and evolution of the cnidosac – the structure that houses sequestered nematocysts (called kleptocnides). In this paper, we describe cnidosac morphology across the main groups of Cladobranchia in which it occurs, and place variation in its structure in a phylogenetic context to better understand the evolution of nematocyst sequestration.

Results

Overall, we find that the length, size and structure of the entrance to the cnidosac varies more than expected based on previous work, as does the structure of the exit, the musculature surrounding the cnidosac, and the position and orientation of the kleptocnides. The sequestration of nematocysts has originated at least twice within Cladobranchia based on the phylogeny presented here using 94 taxa and 409 genes.

Conclusions

The cnidosac is not homologous to cnidosac-like structures found in Hancockiidae. Additionally, the presence of a sac at the distal end of the digestive gland may have originated prior to the sequestration of nematocysts. This study provides a more complete picture of variation in, and evolution of, morphological characters associated with nematocyst sequestration in Cladobranchia.

     

A new species of Armodoris (Mollusca, Gastropoda, Nudibranchia, Akiodorididae) from McMurdo Sound, Antarctica

Recently collected specimens of Armodoris from McMurdo Sound, Antarctica, were morphologically examined and sequenced. Comparison between this new material and literature sources revealed that it belongs to an undescribed species, Armodoris anudeorum. Although this new species is externally very similar to Armodoris antarctica (the only previously known species of Armodoris), these two species differ in several details of their external morphology, and particularly in their reproductive anatomy and radular morphology. This is the second known species of Armodoris; thus, this paper doubles the known diversity of this exclusively Antarctic group.     

Species names and metaphyly: a case study in Discodorididae (Mollusca, Gastropoda, Euthyneura, Nudibranchia, Doridina)

Dayrat, B. & Gosliner, T. M. (2005). Species names and metaphyly: a case study in Discodorididae (Mollusca, Gastropoda, Euthyneura, Nudibranchia, Doridina) —Zoologica Scripta, **, ***–***. Absence of resolution in phylogenetic trees, or metaphyly, is a common phenomenon. It mainly results from the fact that each data set has its own limit and can hardly be expected to reconstruct alone an entire hierarchy. Because metaphyly helps point out which regions of a tree merit further investigation, one should not avoid metaphyly but rather should try to detect it by addressing carefully node reliability. In this paper we explore the implication of metaphyly for species names. We present a phylogenetic analysis of Discodorididae (Mollusca, Gastropoda, Nudibranchia, Doridina), with an emphasis on relationships among species of Discodoris and its traditionally close ‘allies’ such as Peltodoris and Anisodoris. We demonstrate that some species must be transferred to different discodoridid subclades with which they share synapomorphies, but that many species form a metaphyletic group at the base of Discodorididae, and therefore cannot be placed in any taxon of genus level. We demonstrate that the current International Code of Zoological Nomenclature does not allow taxonomists to handle this situation because it requires selecting a taxon name of genus rank for every species binomial. Then we evaluate the results provided by new forms of species names, both in a rank‐based system, such as the current nomenclature, and a rank‐free system. All solutions considered would cause radical changes to the ‘spirit’ of the current ICZN (and, by extension, to the other current codes). In a rank‐free system of nomenclature, such as the PhyloCode, the best result is obtained with an epithet‐based form that does not mention supra‐specific relationships. Under this method, official species names would take the form ‘boholiensis Bergh, 1877’, although page numbers and letters can be added for uniqueness purposes. Taxonomists would then be free to add supra‐specific taxon names in ‘common’ species names, such as Discodorididae boholiensis Bergh, 1877 or simply Discodorididae boholiensis. Here we wish to stimulate discussion of a problem that we believe merits wide debate: absence of resolution in phylogenetic reconstruction and its impact on species nomenclature.     

Initial results on the molecular phylogeny of the Nudibranchia (Gastropoda, Opisthobranchia) based on 18S rDNA data

This study investigated nudibranch phylogeny on the basis of 18S rDNA sequence data. 18S rDNA sequence data of 19 taxa representing the major living orders and families of the Nudibranchia were analyzed. Representatives of the Cephalaspidea, Anaspidea, Gymnomorpha, Prosobranchia, and Pulmonata were also sequenced and used as outgroups. An additional 28 gastropod sequences taken from GenBank were also included in our analyses. Phylogenetic analyses of these more than 50 gastropod taxa provide strong evidence for support of the monophyly of the Nudibranchia. The monophyly of the Doridoidea, Cladobranchia, and Aeolidoidea within the Nudibranchia are also strongly supported. Phylogenetic utility and information content of the 18S rDNA sequences for Nudibranchia, and Opisthobranchia in general, are examined using the program SplitsTree as well as phylogenetic reconstructions using distance and parsimony approaches. 0Results based on these molecular data are compared with hypotheses about nudibranch phylogeny inferred from morphological data.     

Phylogeny of Hypselodoris (Nudibranchia: Chromodorididae) with a review of the monophyletic clade of Indo-Pacific species, including descriptions of twelve new species

This work provides an account of the systematics and phylogeny of Hypselodoris . Aspects of the morphology of 42 species are described and the systematic status of an additional 11 species is discussed. Twelve new species are described: Hypselodoris alboterminata, H. bertschi, H. bollandi, H. fucata, H. iacula, H. insulana, H. krakatoa, H. paulinae, H. reidi, H. rudmani, H. violabranchia and H. zephyra. A phylogenetic analysis supports the monophyly of Hypselodoris and Risbecia . Two distinct clades of Hypselodoris are present. One contains species from the Atlantic and eastern Pacific while the other contains species limited to the Indo-Pacific tropics and adjacent temperate regions. Species from the Atlantic and eastern Pacific are bluish in body colour and have a plesiomorphically large receptaculum seminis while Indo-Pacific taxa are variably coloured and all have a minute receptaculum seminis. The distribution and size of mantle glands provides a wealth of morphological characters. With few exceptions, mantle glands vary in closely related species and are important for distinguishing members of smaller clades. Mantle gland distribution is therefore useful in identifying preserved material that is difficult to identify to species in the absence of the pigment of living specimens. Similar colour patterns found in sympatric species of Hypselodoris appear to be a result of both common descent and convergence between less closely related lineages. Biogeographic distributions of sister taxa provide several examples of vicariance. Examination of these cases shows that no single vicariant pattern is present, but vicariance appears to occur at the margins of the Indo-Pacific rather than centrally. Some vicariance occurs even within archipelagos such as the Hawaiian Islands. These cases largely refute the generality of the hypothesis of Springer (1982), that Pacific Plate and Australasian Plate endemic sister taxa should predominate.     

Ultrastructure and Phylogenetic Significance of Notaspidean Spermatozoa (Mollusca, Gastropoda, Opisthobranchia)

Spermatozoa of five notaspidean opisthobranchs [Berthellina citrina, Berthella ornata, Pleuro-branchus peroni, Pleurobranchaea maculata, Umbruculum sinicum] were examined using TEM. In all five species, the acrosome (sensu lato) consists of an apical vesicle (the acrosomal vesicle) and acrosomal pedestal. The acrosomal pedestal overlaps the nuclear apex, and in P. peroni (and possibly B. ornata) is periodically banded—-the first reported incidence of this type of substructure in any euthyneuran acrosome. Although sperm nuclei of P. peroni, B. ornata and B. citrina differ in length and also the number of keels present (nucleus 7 μm long with four/five keels present in Pleurobranchus; 17 μm long with one keel in Berthella; 15 μm long with a very weak keel in Berthellina), the basal invagination to which the centriolar derivative, axoneme and coarse fibres are attached is always poorly developed, and very little overlap between nucleus and midpiece occurs. In P. maculata and U. sinicum, the nucleus forms a helical cord around the axoneme and mitochondrial derivative such that it is not possible to recognize exclusively ‘nuclear’ and ‘midpiece’ regions of the spermatozoon. In all notaspideans investigated, (1) the axoneme, coarse fibres and glycogen helix are enclosed by the paracrystalline and matrix components of the mitochondrial derivative and (2) a dense ring structure (attached to the plasma membrane) and glycogen piece are observed. While the glycogen piece is very short (0.85–1.43 μm) with a very degenerate axoneme in B. citrina, B. ornata and P. peroni, this region of the spermatozoan is well developed (30–35 μm long) in U. sinicum and exhibits a fully intact 9 + 2 axoneme. The ‘glycogen piece’(or its presumed homologue) in P. maculata spermatozoa is very short (0.65 μm), devoid of any axonemal remnant and constructed of a hollow, internal cylinder attached to an outer (incomplete) shell, and contains scattered (glycogen) granules. Spermatozoal structure supports a close relationship between the genera Berthellina, Berthella and Pleurobranchus. These three genera have more distant links with Pleurobranchaea, while Umbraculum maintains an isolated, specialized position within the Notaspidea.